Stratigraphy of Upper Cenozoic Fluvial Deposits of the La Bajada Constriction Area,

By David P. Dethier and David A. Sawyer

Chapter B of The Cerrillos Uplift, the La Bajada Constriction, and Hydrogeologic Framework of the Santo Domingo Basin, Rift, New Mexico Edited by Scott A. Minor

Professional Paper 1720–B

U.S. Department of the Interior U.S. Geological Survey Contents

Abstract...... 27 Introduction...... 27 Stratigraphic Nomenclature and Correlations...... 28 Fluvial Basin-Fill Deposits...... 28 Lithologic Characteristics...... 28 Isotopic and Amino-Acid-Racemization Ages...... 29 Stratigraphic Framework...... 31 Upper Miocene Through Lower Basin-Fill Deposits...... 31 Pleistocene (Post-Bandelier Tuff) Through Holocene Deposits...... 34 Incisional and Aggradational History of the Ancestral Rio Grande...... 36 Hydrogeologic Implications...... 37 References Cited...... 38

Figures

B1. Sketch map of the Cochiti Pueblo area showing location of axial river gravel deposits, Cerros del Rio lava flows, and selected geologic features...... 29 B2. Diagram showing relation between amino acid racemization ratio in Succinea and deposit age, in thousands of years...... 31 B3. Stratigraphic sections west of the Rio Grande...... 33 B4. Stratigraphic sections near the Rio Grande...... 35 B5. Schematic drawing showing time-position (east-west) distribution of coarse axial river facies deposits and interlayered volcanic rocks in the La Bajada constriction area...... 36 Table B1. Age and height relations of upper Cenozoic alluvium, tephra, and volcanic rocks in the Cochiti Pueblo map area...... 30 Stratigraphy of Upper Cenozoic Fluvial Deposits of the La Bajada Constriction Area, New Mexico

By David P. Dethier and David A. Sawyer

Two large ash-flow tuff sheets of the Bandelier Tuff that Abstract erupted at 1.61 and 1.22 Ma buried upland and valley areas in the constriction. Interlayered sediments and volcanic rocks in Coarse- to medium-grained upper Cenozoic fluvial the northeast part of the Santo Domingo basin are preserved in sediments deposited in the Santo Domingo Basin and adjoin- a complex series of horsts and grabens west of the La Bajada ing La Bajada constriction record the influence of faulting, fault zone and are exposed best along the La Bajada fault-zone volcanism, and climate change on protracted deposition by escarpment and near the village of Cochiti Lake. Coarse- the ancestral Rio Grande and adjacent piedmont drainages. grained alluvium continued to be deposited in the La Bajada Axial sand and gravel deposited by the ancestral Rio Grande constriction area during the Pleistocene by the Rio Grande in were derived from uplands to the northeast and northwest the axial part of the basin and by tributary streams on adjacent of the La Bajada constriction. Interfingering piedmont sand, piedmonts. Faulting, episodes of greatly increased sediment silt, and local gravel were derived from the Sangre de Cristo load, and damming by lava flows and sediment provided Mountains to the east and from the to the controls on sediment deposition. The composite thickness of west. These sediments form part of the , a the Santa Fe Group basin-fill stratigraphic section exceeds sequence of Miocene and younger sediments deposited in a 300 m in the northeast part of the Santo Domingo basin, and series of structural basins along the Rio Grande rift in north- it increases to 3–4 km south of the San Francisco fault in the ern New Mexico. Eastern piedmont deposits of arkosic sand, central part of the basin. Distribution and continuity of late silt, and clay are derived from Precambrian granitic basement Miocene through late alluvial deposits and their rocks and intertongue with coarser axial-river gravel and sand inferred subsurface extent imply mainly basin aggradation derived from granite and distinctive quartzite. These two map- during their deposition. Pleistocene fluvial incision by the pable sedimentary facies are considered informal units within ancestral Rio Grande and its tributaries in White Rock Canyon the Sierra Ladrones Formation, whose type area is to the south removed earlier volcanic dams. Several episodes of aggrada- in the . tion and terrace development punctuated erosion during post- Equivalent upper and middle Miocene piedmont deposits Bandelier time in the La Bajada constriction area, suggesting in the Española Basin to the northeast compose the Tesuque that discharge and sediment load were linked to climatic Formation. Uppermost eastern piedmont sediments in the cycles or to other processes that intensified after early Pleis- Santo Domingo Basin, consisting of sand, silt, clay, and local tocene time. Alluvial terrace fills younger than late middle gravel, may be temporally correlative in part with the Ancha Pleistocene record lateral asymmetry of sediment preservation Formation of the Santa Fe area to the east. Upper Miocene and south of Cochiti Dam; the asymmetry was caused by basinal Pliocene western piedmont deposits, which were derived from tilt owing to fault movements or westward growth of the east- an active volcanic complex in the adjoining Jemez Moun- ern piedmont deposits (or both). tains, are assigned to the Cochiti Formation. Deposition of the Cochiti volcaniclastic sediments postdates emplacement of the 7–6.2 Ma Peralta Tuff Member of the Bearhead Rhyolite. Western piedmont deposits contain more gravel and coarser Introduction sand than the eastern piedmont sediments, and they are domi- nated by compositionally diverse volcanic clasts derived from Studies by the U.S. Geological Survey were begun in the upper Miocene Keres Group of the Jemez volcanic field. 1996 to improve understanding of the geologic framework of Piedmont and axial Rio Grande sedimentary deposits the Albuquerque composite basin and adjoining areas, in order in the La Bajada constriction area are interlayered with and that more accurate hydrogeologic parameters could be applied locally overlie Pliocene and early Pleistocene volcanic rocks. to new hydrologic models. The ultimate goal of this multidis- Multiple lava flows and domes that erupted between 2.7 and ciplinary effort has been to better quantify estimates of future 1.1 Ma from the Cerros del Rio volcanic field were emplaced water supplies for northern New Mexico’s growing urban onto basin-fill sediments in the La Bajada constriction area. centers, which largely subsist on aquifers in the Rio Grande rift 28 Hydrogeologic Framework, La Bajada Constriction Area, Rio Grande Rift, New Mexico basin (Bartolino and Cole, 2002). From preexisting hydrologic composed of volcaniclastic sediments shed from the Jemez models it became evident that hydrogeologic uncertainties were volcanic field. We follow Smith and Lavine (1996) by using large in the Santo Domingo Basin area, immediately upgradient Cochiti Formation (unit QTc) for younger western piedmont from the greater Albuquerque metropolitan area, and particu- deposits in the Santo Domingo Basin that overlie the 7–6.2 larly in the northeast part of the basin referred to as the La Ma Peralta Tuff Member of the Bearhead Rhyolite (unit Tbp). Bajada constriction (see chapter A, this volume, for a geologic Older proximal volcaniclastic deposits of the Jemez volcanic definition of this feature as used in this report). Accordingly, a field are informally named Keres Group volcaniclastic depos- priority for new geologic and geophysical investigations was to its (unit Tkvs). Southwest of Peralta Canyon, upper Pliocene better determine the hydrogeologic framework of the La Bajada piedmont gravel of Lookout Park (unit Tglp) caps Cochiti constriction area. This chapter along with the other chapters of Formation sediments (Smith, McIntosh, and Kuhle, 2001), this report present the results of such investigations as recently which in turn probably overlie unnamed and unexposed older conducted by the U.S. Geological Survey. Miocene basin-fill deposits. Exposed coarse-grained upper Cenozoic fluvial deposits The stratigraphic terminology adopted here is consistent in the La Bajada constriction area record protracted deposition with that used by Connell (2001, 2004) for deposits of similar by the ancestral Rio Grande and adjacent piedmont drain- lithology, age, and origin downstream in the Albuquerque ages that was influenced by faulting, climate change, and Basin. Use of the name Sierra Ladrones Formation in the La lava dams. Axial river gravel and sand are the most abundant Bajada constriction area and adjacent areas of the Rio Grande subsurface aquifer materials in the Rio Grande valley and La rift has recent precedents (Tedford and Barghoorn, 1997; Con- Bajada constriction area (Smith and Kuhle, 1998c, Smith, nell, 2001; Smith, McIntosh, and Kuhle, 2001; Connell and McIntosh, and Kuhle, 2001) and may increase recharge where others, 2002; Koning and others, 2002; Smith, 2004). Some hydraulically connected with underlying aquifers. A thorough disagreements persist, however, about the stratigraphic rank, understanding of the stratigraphic architecture, facies changes, timing, and origin of some Santa Fe Group deposits in the and spatial distribution of fluvial sediments within the La Albuquerque Basin (Connell and others, 1999; Tedford and Bajada constriction and adjacent basins is critical to develop- Barghoorn, 1999; and Williams and Cole, 2005). ing a hydrogeologic framework model of the region. In this chapter, we synthesize the results of geologic mapping and limited subsurface information to describe the stratigraphy and Fluvial Basin-Fill Deposits distribution of upper Cenozoic fluvial basin-fill sediments in the La Bajada constriction area (pl. 2). Lithologic Characteristics Coarse-grained, mainly gravelly, unconsolidated sedi- Stratigraphic Nomenclature and ment of the Santa Fe Group exposed in the La Bajada constric- tion area records the depositional history of the Rio Grande Correlations and its tributary piedmont streams during the late Cenozoic. Gravel clasts in these fluvial deposits were derived from three Fluvial sand and gravel deposited by the ancestral Rio main source areas: (1) quartzites, metavolcanic rocks, chert, Grande and piedmont sediments derived from uplands to the and other high-grade Precambrian metamorphic rocks were northeast and northwest of the La Bajada constriction are transported from the northwest by the ancestral Rio Chama correlated broadly with the Santa Fe Group (“formation” of and from the north by the ancestral Rio Grande; (2) intermedi- Bryan and McCann, 1937, and Bryan, 1938; Santa Fe Group ate, silicic, and minor basaltic rocks were transported from the in Spiegel and Baldwin, 1963; Tedford, 1981; and Chapin west and northwest by piedmont streams draining the Jemez and Cather, 1994). In this report, we adopt the stratigraphic Mountains; and (3) granitic rocks, high-grade metamorphic terminology of Smith, McIntosh, and Kuhle (2001) for the rocks, and minor metasedimentary rocks were transported upper Santa Fe Group deposits in the Santo Domingo Basin. from the east and northeast by the ancestral Santa Fe River and They used the name Sierra Ladrones Formation (of the Santa its tributary piedmont streams. Fe Group) (Machette, 1978) for ancestral Rio Grande axial Ancestral Rio Grande axial-gravel deposits of the Sierra river gravel and sand deposits rich in granite and quartzite Ladrones Formation (unit QTsa, pl. 2) are coarse grained— clasts (unit QTsa; unit abbreviations are those of pl. 2 of this generally sand to cobble and boulder gravel—and commonly report) together with intertonguing eastern piedmont facies are well sorted in surface exposures. Matrix of the gravel is derived from granitic basement rocks (unit QTsp). Upper- generally coarse arkosic sand. Lenses and local lenticular beds most Miocene(?) to Pleistocene eastern piedmont sand, mud, of fine sand and silty sand locally are rich in altered pumice. and local gravel (unit QTsp) correlate in part with the Ancha Piedmont alluvium flanking the axial gravel locally Formation and underlying in the Espa- is also coarse grained but commonly contains more sand ñola Basin (Spiegel and Baldwin, 1963; Kuhle and Smith, and silty sand and is more poorly sorted than the axial 2001; Koning and others, 2002). In the La Bajada constriction river alluvium. Some of the poorly sorted piedmont beds area, the western piedmont facies of the Santa Fe Group is are debris-flow deposits. Western piedmont alluvium of Upper Cenozoic Fluvial Deposits, La Bajada Constriction Area, New Mexico 29 the Cochiti Formation (unit QTc) contains abundant clasts 106° 22’ 30” 106° 15’ 35°45’ derived from the ~7-Ma Bearhead Rhyolite (unit Trb) and e White d n a fewer clasts of older Keres Group andesites, rhyolites, and Rock r G Canyon o dacites (Smith and Kuhle, 1998c). Pumice-rich intervals and Ri Sanchez interlayered lavas help to constrain the age of the piedmont Canyon alluvium (Smith, McIntosh, and Kuhle, 2001). In exposures Dome near Cochiti Lake, eastern piedmont alluvium of the Sierra Road Ladrones Formation (unit QTsp) is rich in granitic debris Bland Canyon 40 57 and is interlayered with basalt flows and with lacustrine, 74 Peralta Canyon eolian, and local basaltic hydromagmatic deposits of the Cochiti Reservoir Cerros del Rio volcanic field. 78 7 27 Most piedmont and axial alluvial deposits observed beneath Quaternary fluvial terraces and between volcanic units Cochiti 26 X Dam have exposed thicknesses of 5–30 m. Total exposed thickness 80 Santa Cañon Santo Cruz of interstratified alluvium of the Rio Grande (unit QTsa) and Springs Domingo Sa western piedmont gravel (unit QTc), however, locally exceeds nta Tract Fe e R 70 m along Cochiti Canyon and in the southwestern part of d iv n e a r the map area (pl. 2). In some areas these fluvial deposits are r

G

exposed for distances >1 km. Near Cochiti Dam, the strati- o i graphic thickness of ancestral Rio Grande deposits (unit QTsa) R Southern extent, is interpreted to locally exceed 350 m (Smith, McIntosh, basalt of and Kuhle, 2001). The Dome Road well, about 4 km west of Pena Blanca Cochiti Lake, penetrated nearly 400 m of Cochiti Formation Ga liste western piedmont alluvium (Chamberlin, Jackson, and Con- o Creek nell, 1999; chapter G, this volume). 35°30’ Contact relations and lateral extent of the Santo 0 2 KILOMETERS Domingo Basin alluvial deposits are known mainly from surface exposures. These exposures suggest that upper Santa EXPLANATION Fe Group sediments were deposited mostly as a conformable sequence during late Miocene to early Pleistocene (that is, Cerros del Rio lava flows 1.61 Ma) time. Although episodes of downcutting by the Rio Grande during this time period produced intraformational Exposed axial sand and gravel deposits disconformities, large angular unconformities were not rec- 78 Stratigraphic sections noted in text ognized in the upper part of the basin-fill sequence by Smith, Wells noted in text McIntosh, and Kuhle (2001). The stratigraphy of the Santa X Axial gravel outcrop Fe Group in the La Bajada constriction is similar to that of the adjoining Albuquerque and Española Basins, in that basin filling continued episodically until early Pleistocene (that Figure B1. Cochiti Pueblo area and vicinity showing location is, post-Bandelier Tuff) time. However, Williams and Cole of axial river gravel deposits, Cerros del Rio lava flows, selected (2005) describe a prominent unconformity beneath Pliocene geologic features, and stratigraphic sections noted in text and rocks in the Albuquerque Basin and, in the Española Basin shown in figures B3 and B4. and adjacent White Rock Canyon (pl. 1, fig. B1), prominent disconformities separate upper Miocene rocks from Pliocene units and separate Pliocene strata from the Bandelier Tuff (Reneau and Dethier, 1996).

Isotopic and Amino-Acid-Racemization Ages age for the oldest river gravels in the study area as well as for the Cochiti Formation (unit QTc), which is derived Local stratigraphic relations, 40Ar/39Ar dating of in part from the Bearhead Rhyolite. A radiometric age of basaltic rocks and silicic tuffs, 14C ages of organic material 6.82 Ma for rhyolitic pumice within river gravel (table B1) in sediments, and amino-acid-racemization ratios (AAR provides additional local age control for upper Miocene ratios) for gastropod fossils in Quaternary deposits provide basin-fill deposits in the La Bajada constriction area (Smith age control (table B1) in the northeast part of the Santo and Kuhle, 1998a,b; Smith, McIntosh, and Kuhle, 2001). Domingo Basin. The Bearhead Rhyolite (unit Trb, pl. 2) and No radiometric ages exist for alluvium or volcanic rocks its Peralta Tuff Member (unit Tbp), dated at 7–6 Ma (Smith, deposited between about 6 and 3 Ma in the northeast part of McIntosh, and Kuhle, 2001), provide a minimum limiting the Santo Domingo Basin. 30 Hydrogeologic Framework, La Bajada Constriction Area, Rio Grande Rift, New Mexico

Table B1. Age and height relations of upper Cenozoic alluvium, tephra, and volcanic rocks in the Cochiti Pueblo map area. [40Ar/39Ar ages from Smith and others, 2001; W.C. McIntosh, personal commun., 1997; Izett and Obradovich, 1994 (Guaje Pumice Bed); and Sarna-Wojcicki and others, 1987. 14C ages from Reneau and Dethier, 1996; Reneau and others, 1996; and Lanphere and others, 2002. Height above Rio Grande measured from the active channel (or former active channel where inundated by Cochiti Reservoir) to the top of gravel or base of volcanic flow. ND, not determined]

Height Map Dating method above unit Age and surficial or bedrock unit Rio Location and stratigraphic context symbol (k.y.) (pl. 2) Grande (pl. 2) (m) 40Ar/39Ar Peralta Tuff Member of Bearhead Rhyolite. Tbp 6,840–6,810 ND Interlayered with axial gravel, Tent Rocks area. Pumiceous alluvium in Rio Grande axial Rhyolite tephra interlayered with Sierra Ladrones Formation gravel. QTsa 6,880–6,820 ND axial gravel west of Cochiti Pueblo. Rhyolite tephra, basal Cochiti Formation QTc 6,190 ND Tephra in basal Cochiti Formation, Tent Rocks area.

Basalt of Pena Blanca Tbpb 2,710 40 Pena Blanca flow overlies axial alluvium.

Basalt of White Rock Canyon Tb 2,550 30 Overlies axial gravel east and west of Cochiti Reservoir. Pumice from rhyolite of San Diego Canyon; underlies Pumiceous alluvium in Cochiti Formation QTc 1,850 45–55 Otowi (lower) Member of Bandelier Tuff. Beneath Otowi Member of Bandelier Tuff near Rio Grande alluvium QTsa 1,610 95–100 La Bajada escarpment. Overlies Guaje pumice bed and pumiceous alluvium east of Basalt of Cochiti QTbc 1,460 75–85 Cochiti Reservoir. Overlies Otowi Member of Bandelier Tuff, north of La Dacite of Arroyo Montoso Qda 1,310 210 Bajada fault zone. Tshirege (upper) Member of Bandelier Tuff. Qbt 1,220 90–100 In paleochannel along Rio Grande. Overlies axial alluvium containing clasts of upper, Basaltic andesite of Cochiti Cone Qcc 1,140 75? Tshirege Member of Bandelier Tuff.

Oldest alluvial terrace deposits of 1 Axial Rio Grande sediments containing Lava Creek B Qta1 1,220–639 75 Rio Grande. ash, beneath Qta1, west of Santo Domingo Pueblo area. Old alluvial terrace deposits of Qta 550 58 Reworked Valles Rhyolite pumice in Qta fill. Rio Grande. 2 2 Amino-acid racemization ratios

Old alluvial terrace deposits of Qta 2250–300 60–70 From gastropods at two sites. Rio Grande. 2 Intermediate alluvial terrace deposits of Qta 60–300? 38–48 Probably late middle Pleistocene. Rio Grande. 3 14C, thermoluminescence, and electron spin resonance.

3 El Cajete tephra Qec 48–61 15(?) Near level of Qta4 terrace.

14C and amino-acid racemization Young alluvial terrace deposits of Sediment on Qta terrace; amino-acid racemization Qta 4>38 15–18 4 Rio Grande. 4 analyses suggest age of 70–30 ka. 14C Base of overlying lacustrine sediment, northern Older alluvial deposits of Rio Grande Qoa ≥43.5 15 White Rock Canyon. Older alluvial deposits of Rio Grande Qoa 14–12 5–12 Interbedded with lacustrine sediment, White Rock Canyon. Channel and floodplain deposits of Qal 9 5 Central White Rock Canyon. Rio Grande.

1In Española Basin north of White Rock Canyon, Lava Creek B ash is ~110 m above Rio Grande (Dethier and others, 1990); in lower Jemez River drainage, ash is ~95 m above grade (Rogers and Smartt, 1996). 2Based on hydrolysate aIle/Ile ratios (amino-acid racemization) from Succinea or Vallonia from stratigraphic sections DN–96–26 and DN–96–80 (Dethier and McCoy, 1993). 3Carbonized logs in El Cajete tephra yield 14C ages of 50 to >58 ka, and thermoluminescence ages on buried soils give age estimates 48–61 ka (Reneau and others, 1996). Electron- spin-resonance dating of stable Ti centers in quartz provides ages of 53 ka for the El Cajete tephra and 59 ka for the genetically related Battleship Rock ignimbrite (Toyoda and others, 1995). 4Shell dates are based on hydrolysate aIle/Ile ratios (amino-acid racemization) from Succinea and Vallonia from locality DN–96–18 of Dethier and McCoy (1993) and are assumed to be minimum limiting ages. Upper Cenozoic Fluvial Deposits, La Bajada Constriction Area, New Mexico 31

Age control for Pliocene-Pleistocene alluvial deposits was deposited between about 70 and 30 ka, which is consis- is based mainly on locally derived volcanic rocks. Basaltic tent with minimum limiting 14C ages of >43.5 and >38 ka for hydromagmatic volcanism and deposition of associated terrace fills at approximately the same height above the Rio deposits (unit Tbm, pl. 2) centered near the modern White Grande elsewhere (table B1). Rock Canyon (pl. 1, fig. B1) may have begun as early as 2.9

Ma (Reneau and Dethier, 1996), and basalt flows and basaltic 700 tuff (units Tbj, Tbb, Tbs) erupted soon after at about 2.7 Ma from several vents along the southern and eastern flanks of the 600 Cerros del Rio volcanic field. Pliocene axial gravels of the Rio Grande (unit QTsa, 500 pl. 2) underlie basalt flows (units Tb, Tbf) dated at about 2.5 Ma (table B1) where exposed at several locations along the 400 margins of Cochiti Reservoir. Early Pleistocene gravelly allu- 300 vium (unit Qog) overlies some of these flows (Dethier, 1999). Española Basin Latest Pliocene and early Pleistocene stratigraphic control of the western piedmont (unit QTc) and axial river sequences 200 is provided by tuff derived from the rhyolite of San Diego AGE, IN THOUSANDS OF YEARS 100 Northeast part of the Santo Canyon (about 1.85 Ma) and by the widespread Otowi (lower, 50 Domingo Basin (estimated) unit Qbo) Member (1.61 Ma) and Tshirege (upper, unit Qbt) 0 Member (1.22 Ma) of the Bandelier Tuff (table B1) (Izett and 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Obradovich, 1994). Several dated and a number of undated aIle/Ile, TOTAL HYDROLYSATE (AAR) RATIO flows composed of andesite, trachyandesite, and dacite are EXPLANATION younger than the Otowi Member, and one andesite flow (unit Jemez River valley Qcc) overlies the Tshirege Member; these flows provide local stratigraphic control for early Pleistocene deposits in the east- Española Basin ern part of the La Bajada constriction area. The Lava Creek B Cochiti Dam area ash (639 thousand years old (ka); Lanphere and others, 2002) and layers of reworked pumice from the Valles Rhyolite (550 Figure B2. Relation between amino acid racemization (AAR) ka; rhyolite of South Mountain or rhyolite of San Antonio ratio in Succinea and deposit age, in thousands of years. Mountain) are interbedded locally with middle Pleistocene Parabolic curve for northeast part of the Santo Domingo Basin terrace deposits (units Qta1, Qta2; Smith, McIntosh, and estimated from curve for the Española Basin (Dethier and McCoy, Kuhle, 2001). 1993). Analyses from the Jemez River valley are from Rogers and Approximate ages calculated from AAR ratios for Smartt (1996). Data points with horizontal arrows pointing toward gastropod fossils (fig. B2) and 14C ages provide age limits for parabolic curve derived from AAR values reported by Rogers and middle to late Pleistocene deposits (table B1). Several gas- Smartt (1996) for diagenetically altered samples. tropod genera collected at 7 sites from 2 alluvial fill terraces of the Rio Grande (units Qta2 and Qta4; pl. 2) were analyzed by using techniques described by Dethier and McCoy (1993). We derived a preliminary AAR age relation using the genus Stratigraphic Framework Succinea from the La Bajada constriction area; the relation is based primarily on the parabolic racemization curve of Upper Miocene Through Lower Pleistocene Dethier and McCoy (1993), which has been modified for the Basin-Fill Deposits slightly higher temperatures in the Cochiti Dam area (fig. B2). Values for AAR ratios obtained on samples from the nearby Alluvial and piedmont-fan sand and gravel basin-fill sed- and slightly cooler valley of the Jemez River plot close to iments deposited during the Miocene are exposed in the La the Española Basin curve except for the two oldest samples, Bajada constriction area. Upper Oligocene–lower Miocene(?) which are altered (fig. B2) (Rogers and Smartt, 1996). The ashy siltstones that crop out along the La Bajada escarpment,

AAR data suggest that the Qta2 terrace fill was deposited 10 km east-southeast of Cochiti Reservoir near the village about 300–250 ka (table B1). These AAR ages seem incon- of La Bajada (pl. 1), may correlate with the Abiquiu(?) sistent with a 40Ar/39Ar date of 550 ka for reworked Valles Formation (pl. 2) (Stearns, 1953; Sawyer and others, 2002).

Rhyolite pumice in another deposit mapped as unit Qta2. If the These siltstones and younger basin-fill deposits cannot be reworked pumice provides a close limiting age for the fill and traced north and west from the La Bajada exposures because if the AAR ages are correct, then the Rio Grande may have massive landslide deposits and extensive colluvium mantle remained approximately 60 m above present grade for 200 pre-basalt geologic units along the La Bajada escarpment. To thousand years (k.y). or backfilled to that height after post- the east of the escarpment, Pliocene to middle Pleistocene

550 ka downcutting. AAR ages show that unit Qta4 terrace fill piedmont deposits of the Ancha Formation and the Tuerto 32 Hydrogeologic Framework, La Bajada Constriction Area, Rio Grande Rift, New Mexico

Gravel (unit QTt, pl. 2) overlie Oligocene and older rocks gravel deposits near the west edge of the map area (pl. 2). The that are described in chapter A (this volume). On the western gravel of Lookout Park, in turn, is locally overlain by erosional margin of the La Bajada constriction, gravel-rich deposits of remnants of early Pleistocene Otowi Member of the Bandelier the Cochiti Formation are interlayered with and overlie the Tuff (unit Qbo). South of Cochiti Dam, ancestral Rio Grande upper Miocene Bearhead Rhyolite (unit Trb). Miocene Santa axial gravel and sand (units QTsa, Tsls) interfinger with eastern Fe Group basin-fill deposits are inferred to lie beneath the piedmont alluvium (unit QTsp) as much as 5 km east of the central part of the La Bajada constriction but are nowhere modern Rio Grande (fig. B1). Farther south, on the north side exposed. Below we discuss the upper Miocene through Pleis- of Galisteo Creek, axial river sand (unit Tsls) underlies eastern tocene basin-fill deposits in more detail. piedmont alluvium, which in turn underlies the Otowi Member We divide upper Miocene, Pliocene, and early Pleistocene of the Bandelier Tuff (pl. 2) (Smith and Kuhle, 1998b). basin-fill deposits into three stratigraphic units that consist of Ages of capping basalt flows (unit Tb, pl. 2) indicate that the sedimentary facies described earlier: (1) Cochiti Forma- Rio Grande axial gravel is older than about 2.5 Ma in lower tion western piedmont alluvium (unit QTc, pl. 2); (2) Sierra White Rock Canyon in the northern part of the map area (table Ladrones Formation axial river gravel (unit QTsa); and (3) B1). In the upper part of this canyon about 15 km north of Sierra Ladrones Formation eastern piedmont alluvium (unit Cochiti Reservoir, basalt flows as old as 2.9 Ma overlie coarse QTsp). These units include (1) deposits coeval with or older Rio Grande alluvium (Reneau and Dethier, 1996). To the than the approximately 2.7–2.5-Ma Cerros del Rio basalt flows south in the Santo Domingo Valley (pl. 1) the 2.7-Ma basalt (units Tbb, Tbj, Tbs, Tb); (2) Pliocene and Pleistocene deposits of Peña Blanca (unit Tbpb) overlies >20 m of Rio Grande younger than the basalts but older than the 1.22-Ma Tshirege axial sand (unit Tsls) that interfingers with eastern piedmont (upper) Member of the Bandelier Tuff (unit Qbt); and (3) alluvium (unit QTsp) (pl. 2). Cuttings from the Santa Cruz deposits that are present principally in the La Majada graben Springs Tract well (figs. B1, G1, G3) suggest that at least 30 (fig. A4) that are probably younger than Cerros del Rio basalt m of axial river gravel, interbedded with eastern piedmont but older than the <600 ka alluvium of La Majada Mesa (unit alluvium, overlies a 2.57–2.55-Ma basalt at depth (Smith and Qalm) and its associated erosion surface. Smith and Kuhle Kuhle, 1998a; Smith, McIntosh, and Kuhle, 2001; G.A. Smith, (1998a,b) mapped alluvial deposits older than early Pleistocene written commun., 2001). This interpretation is consistent with as Cochiti Formation (western piedmont facies) and Sierra the evidence described above that the ancestral Rio Grande Ladrones Formation (Rio Grande axial river and eastern pied- flowed at least 5 km east of its present course during middle mont facies); they mapped younger Pleistocene deposits as a Pliocene time. Along the western shore of Cochiti Reservoir, sequence of alluvial terrace deposits. Rio Grande axial gravel (unit QTsa) is exposed beneath basalt Axial river gravel and sand deposits older than middle flows (unit Tb) dated at about 2.5 Ma (stratigraphic section Pliocene (unit QTsa) have been mapped in many places in the DN–96–7, fig. B3C). La Bajada constriction area (pl. 2). Peralta Tuff Member of The thickest and most extensive exposures of western the Bearhead Rhyolite (unit Tbp; 6.88–6.81 Ma) interlayered piedmont deposits (Cochiti Formation) and interfingering with coarse axial river gravel and sand deposits in the Tent ancestral Rio Grande alluvium (Sierra Ladrones Formation Rocks area (pl. 2) demonstrate that a river transporting clasts axial river gravel facies) are near, west, and southwest of from northern sources flowed through the western part of the Cochiti Dam (pl. 2), along deeply dissected drainages such as map area as early as latest Miocene time (Smith and Kuhle, those in lower Bland Canyon and near stratigraphic section 1998a,b,c; Smith, McIntosh, and Kuhle, 2001). Along the La DN–96–78 (figs. B1, B3A). As indicated above and discussed Bajada escarpment, at the mouth of Tetilla Arroyo (pl. 1), a in more detail in chapter G (this volume), the Dome Road >30-m-thick section of axial river gravel crops out beneath well penetrated 400 m of Cochiti Formation western piedmont basalt flows (unit Tbf) dated at about 2.5 Ma (see “X” in fig. deposits (fig. G5). The Dome Road well did not encounter B1). This outcrop, on the upthrown side of the La Bajada axial-gravel-facies alluvium and thus provides a local western fault zone (pl. 2; fig. B1), provides evidence that the ancestral limit for the Pliocene ancestral Rio Grande. Rio Grande deposited gravel east of the fault zone and at Silt and clay-rich lacustrine deposits (units Tsll, Tslm, least 5.5 km east of the modern Rio Grande channel before pl. 2) exposed locally north of Galisteo Creek are intercalated the middle Pliocene. The eastern limit of Rio Grande axial with axial sand deposits (unit Tsls) and underlie 2.7–2.55-Ma river gravel older than middle Pliocene is poorly constrained basalts (unit Tbpb). The lacustrine deposits are several meters beneath lava flows of the Cerros del Rio volcanic field (pl. 2; thick and contain laterally extensive diatomite layers. Fossil also see chapter G, this volume). diatom assemblages suggest that climate at the time of their South of Peralta Canyon, ancestral Rio Grande axial gravel deposition was cooler than at present and that the lowland (unit QTsa) is moderately exposed throughout a broad area that landscape was forested, perhaps similar to the modern Jemez extends as far as about 12 km west of the modern Rio Grande Mountains (J.P. Bradbury, U.S. Geological Survey, written (pl. 2; fig. B1). In this area the gravel interfingers with pied- commun., 1996). Other clay-rich lacustrine deposits are inter- mont deposits of the Cochiti Formation (unit QTc). A mantle of preted on the basis of geophysical and well data to underlie the younger western piedmont gravel of Lookout Park (unit Tglp; Otowi Member of the Bandelier Tuff east of the Pajarito fault Smith and Kuhle, 1998a,b,c) locally overlies the axial river zone (chapters F and G, this volume). Upper Cenozoic Fluvial Deposits, La Bajada Constriction Area, New Mexico 33

A. Section DN–96–78 B. Section DN–96–57

Graphic log Description Graphic log Description Terrace gravel; stage III carbonate 1,733 Formation

Terrace gravel; Cochiti Buff mudstone; gray pumiceous stage III carbonate sand contains rhyolite of San Gray pumiceous sand; Diego Canyon tephra; volcaniclastic gravel 1,715 andesitic(?) gravel Cobble gravel; subangular basaltic and andesitic clasts

Pumiceous silty sand; gray, green Cochiti Formation fine beds 1,725 1,710 Cochiti Formation Medium sand to cobble and axial gravel gravel rich in volcanic rocks from the Jemez Mountains; local massive Layers of cobble- to boulder-sized beds of pebbly silty sand andesitic clasts, many angular; and interlayered axial some massive beds of pebbly gravel 1,715 sand 1,700 ELEVATION, IN METERS ELEVATION,

ELEVATION, IN METERS ELEVATION, Contact gradational or interlayered Formation and axial gravel Interbeded Cochiti

Axial gravel 1,705 1,690

Axial gravel interlayered Axial gravel and beds with beds of sand and of mixed axial gravel silty sand and western piedmont alluvium; local beds of sand and silty sand

1,695 1,680

C. Section DN–96–7 Graphic log Description

Qta2 terrace 1,660 Cobble gravel, rich in quartzite and basalt Piedmont and axial gravel; buried soils

Figure B3. Stratigraphic sections west of the Rio Grande; Piedmont gravel; contains 0.55-Ma locations (last number of section designation) shown in fig. B1. Valles Rhyolite pumice A, Graphic log and description of stratigraphic section DN–96–78. Boulder gravel rich in intermediate volcanic B, Graphic log and description of stratigraphic section DN–96–57. 1,650 rocks; contains sparse clasts of basalt C, Graphic log and description of stratigraphic section DN–96–7. and upper Bandelier Tuff; paleocurrent direction 220°

Basalt—2.52±0.04 Ma ELEVATION, IN METERS ELEVATION, Thin-bedded, light-colored silt and 1,640 fine sand; diatomite layers rich in Fragilaria virescens Imbricated axial gravel; base not exposed 34 Hydrogeologic Framework, La Bajada Constriction Area, Rio Grande Rift, New Mexico

Upper Pliocene–lower Pleistocene deposits and volcanic fault zone. Sediment fill that dammed the river was derived rocks in the La Bajada constriction area include Rio Grande from eastward-flowing streams south of Capulin Canyon older gravelly alluvium (unit Qog, pl. 2), western piedmont (pl. 1); it is composed of tuff blocks eroded from the lower sediments of the uppermost Cochiti Formation (unit QTc), part of White Rock Canyon by the Rio Grande. Stratigraphic older alluvium of Cerro Toledo Rhyolite interval (unit Qact), (stratigraphic section DN–96–26, fig. B4C) and geomorphic Bandelier Tuff (units Qbo, Qbt), and some younger basaltic relations north of the Cochiti Pueblo map area suggest that flows from the Cerros del Rio volcanic field. North of Galisteo even after the dam failed, post-1.22-Ma resistant basaltic lava Creek, on the south flank of La Majada Mesa, uppermost flows impeded headward incision by the Rio Grande through (early to middle Pleistocene) Santa Fe Group eastern piedmont White Rock Canyon until middle Pleistocene time (Reneau deposits (unit QTs) can be identified locally where the 1.61- and Dethier, 1996). Ma Otowi Member of the Bandelier Tuff (unit Qbo) separates La Majada Mesa is a prominent geomorphic feature them from underlying eastern piedmont deposits (unit QTsp). bounded on the west by the Rio Grande, on the south by Gali- Rio Grande axial gravels (unit QTsa) of Pliocene-Pleistocene steo Creek, and on the east by the La Bajada fault zone (pl. 2). age crop out near and east of the modern Rio Grande (pl. 2) The mesa is capped east of Cochiti Dam by eolian sand and (G.A. Smith, written commun., 2001). Northwest and north- at least 10–12 m of underlying sandy alluvium with gravelly east of Cochiti Dam, thin piedmont gravels of the Cochiti lenses that is informally called the alluvium of La Majada Formation (stratigraphic section DN–96–57, fig. B3B) contain Mesa (unit Qalm). This alluvium contains numerous buried rhyolite of San Diego Canyon tephra (about 1.85 Ma) and fill soils that suggest a history of alternating landscape stability southeast-trending paleochannels that underlie remnants of and accretion. The uppermost soil has an eroded Bt horizon the Otowi Member of the Bandelier Tuff. These stratigraphic above a stage III to IV calcic horizon. Locally the upper soil relations demonstrate that during latest Pliocene or earliest lacks a Bt horizon and has a stage II to III calcic horizon. The Pleistocene time, streams depositing western piedmont gravel alluvium of La Majada Mesa overlies an extensive, gently flowed east of Cochiti Reservoir into an area that later (during west-sloping (about 0.2°–2°), erosion surface formed on upper middle Pleistocene time) received eastern piedmont gravel Santa Fe Group sediments (unit QTs). This surface likely was and alluvial-fan deposits (Smith, McIntosh, and Kuhle, 2001). graded to the Rio Grande when it formed, but it now projects Transport and deposition of sediment on piedmont slopes con- to about 75 m above the present river. The alluvium of La tinued after eruption of the Otowi Member of the Bandelier Majada Mesa is similar in age to, or slightly younger than, the Tuff; the resulting piedmont deposits in the central La Bajada “La Bajada pediment” of Kirk Bryan, the intermediate of his constriction area (unit Qact, for example) are relatively thin three high erosion surfaces graded to the Rio Grande (Bryan and fill channels cut <15 m deep into the underlying tuff. and McCann, 1938). The alluvium of La Majada is possibly correlative in part with the “Ridge terrace” of Aby (1997) and

with the axial-gravel terrace (unit Qal1) of Dethier (1999), Pleistocene (Post-Bandelier Tuff) Through which has an age of about 650–550 ka (Dethier and McCoy, Holocene Deposits 1993) but locally includes sediments almost as old as the upper Bandelier Tuff. Geomorphic and stratigraphic relations suggest that Younger alluvial fill–terrace deposits (units Qta2, Qta3, several episodes of aggradation punctuated fluvial incision Qta4) are inset within and below the unit Qta1 terrace deposits during post-Bandelier time. After eruption of the Tshirege near the modern Rio Grande channel (pl. 2) (Dethier, 1999; (upper) Member of the Bandelier Tuff (unit Qbt) at 1.22 Ma, Smith, McIntosh, and Kuhle, 2001). These alluvial deposits are the Rio Grande migrated to near its present position, deposited composed of axial river gravel as thick as 30 m and are overlain a complex alluvial terrace fill sequence, and incised to its pres- by, and locally interfinger with, sandy alluvium derived from ent elevation (Dethier, 1999). Deposits that form the highest both eastern and western piedmonts. Early(?) Pleistocene and and thickest alluvial terraces (unit Qta ) may record extremely 1 younger piedmont terrace alluvial deposits (units Qtp1, Qtp2, rapid erosion of the Tshirege Member of the Bandelier Tuff Qtp3, and Qtp4) are preserved from the Cochiti Reservoir (unit Qbt) in the lower part of White Rock Canyon (pl. 1, fig. area southward to the main part of the Santo Domingo Basin. B1) and its tributary valleys (stratigraphic section DN–96–40, Local stratigraphic relations of the older Rio Grande alluvial fig. B4D). The outcrop pattern of these early(?) Pleistocene fill-terrace deposits and gravelly alluvium (units Qog, Qta1) alluvial terrace deposits (unit Qta1) is fan-shaped (pl. 2) are obscure at many sites because of the similar composition (Dethier, 1999). Thick (10–30 m) alluvial deposits beneath the and size of gravel clasts (stratigraphic section DN–96–27, fig. unit Qta1 terrace surfaces contain 3–5-m blocks of upper Ban- B4A). Age estimates (table B1) suggest that unit Qta2 deposits delier Tuff (unit Qbt), coarse boulder gravel from the western aggraded between about 550 and 300 ka and that extensive piedmont, and subordinate amounts of reworked older axial younger fill–terrace deposits (units Qta3, Qta4) are middle and river gravel (unit QTsa). The outcrop pattern and composition late Pleistocene in age. In the vicinity of Cochiti Lake, the Rio of the unit Qta1 alluvial terrace deposits suggests that they Grande cut down tens of meters before aggradation of the unit formed after the Rio Grande had breached a natural dam at Qta2 fill, which in turn was followed by additional substantial the constriction near the northwestern end of the La Bajada downcutting. Extensive deposits of unit Qta1 and unit Qta2 crop B ELEVATION, IN METERS ELEVATION, IN METERS A . SectionD . SectionD 1,630 1,670 1,690 1,650 1,690 1,700 1,680 1,710 1,670 Graphic log Qta 1 Graphic log surface Covered Qta N N 1 terrace –9 –9 6 6 –74 –27 Pumiceous sandandgravel; Imbricated axialgravel(paleocurrent Imbricated bouldergravel(>2m); Imbricated axialgravel; Basalt—2.55 to0.04Ma Reddish sandygravel;contains Description Axial gravelatlakelevel Angular basaltboulders;matrix Bandelier Tuff pumice stage IIIcarbonateinburiedsoil and lensesofreworkedupper direction 200°);containsbeds 1 km rocks; exposurecontinuousfor gravel andsubangularvolcanic Bandelier Tuff; matrixaxial basalt andsparseupper mainly intermediatevolcanics, cemented Formation wherelocally and buriedsoils;calledCochiti pumice fragmentsandlenses, carbonate incappingsoil axial gravel;localstageIV mixed piedmontalluviumand poorly exposed Description Layer ofcoarseboulders, Imbricated axialgravel Upper BandelierTuff; Boulder gravelcontaining Surface richinangularto Cobble tobouldergravel; 5 mblocksofupper some rounded boulders subangular basaltic overlies thinashfallunit volcanic rockclasts rich inintermediate Bandelier Tuff Upper Cenozoic Fluvial Deposits, La Bajada Constriction Area, New Mexico D ELEVATION, IN METERS . SectionD 1,710 1,720 1,690 1,700 1,730 D C B A (last numberofsectiondesignation) showninfig.B1. Figure B4. , Graphicloganddescriptionof stratigraphicsectionDN–96–26. , Graphicloganddescriptionof stratigraphicsectionDN–96–40. , Graphicloganddescriptionof stratigraphicsectionDN–96–74. , Graphicloganddescriptionof stratigraphicsectionDN–96–27. SW C ELEVATION, IN METERS . SectionDN−96−26 1,665 1,645 1,655

N Stratigraphic sectionsneartheRioGrande;locations –9 Guaje PumiceBed Qta Graphic log Qta 6 Graphic log 1 Vertical exaggeration~10:1 terracedeposits 2 terrace – 40 Imbricated, locallychannel–crossstratified Mixed axialgravelandpiedmontsand; Imbricated, locallychannel-crossstratified Eastern piedmontgravelabovegravellysand Description upper BandelierTuff axial gravel;includesclastsofreworked and siltlenses;sparsegastropods sand, paleocurrentdirections60˚to110˚ axial gravel;lensesofalteredpumiceous NE Description Middle Pliocenebasalt; Boulder gravel(Western Lower BandelierTuff Massive axialgravel; Gravel; containsboulders Western piedmontgravel gravel interlayered withaxial piedmont) Tuff of upperBandelier section below axial gravelin basalt boulders ofvesicular contains rounded

35 36 Hydrogeologic Framework, La Bajada Constriction Area, Rio Grande Rift, New Mexico out north of Cochiti Dam, but alluvial deposits of early(?) to Rio Grande W Valley E middle Pleistocene age are poorly preserved south of the dam 0 (pl. 2). Near and south of Cochiti Dam, middle and late Pleisto- Upper and lower Cerros del Rio Bandelier Tuff lava flows cene alluvial terrace deposits (units Qta3, Qta4) crop out mainly east of the Rio Grande, where they are covered by thin depos- 2 Cerro Toledo San Diego Canyon tuff tuffs its of eastern piedmont alluvium (units Qtp3, Qtp4, Qpu) and locally overlie older axial river gravel and sand (units QTsa, Cochiti 4 Formation, Axial Eastern Tsls). The youngest extensive fill-terrace deposits (unit Qta3) western piedmont river gravel piedmont are preserved mainly between Cochiti Dam and Galisteo Creek, AGE, IN Ma volcaniclastic facies alluvial east of the Rio Grande (pl. 2) (Smith and Kuhle, 1998a). Smith, facies facies 6 McIntosh, and Kuhle (2001) attribute the lateral asymmetry Bearhead (that is, eastern dominance) of younger alluvial fill remnants Rhyolite Santa Fe Group ? (pre-axial drainage) to middle and late Pleistocene migration of faulting and stream 8 deposition toward the eastern side of the Santo Domingo Basin. Holocene fluvial deposits (unit Qal, pl. 2) form the flood- Figure B5. Time-position (east-west) distribution of coarse axial plain and low terraces along the Rio Grande and its tributaries river facies deposits and interlayered volcanic rocks in the La south of Cochiti Dam. North and east of the dam, undifferenti- Bajada constriction area. Width of drawing represents lateral ated late Pleistocene and Holocene gravelly alluvial fan depos- (east-west) distance of approximately 20 km. its (unit Qfa) flank tributary arroyos, but axial river deposits are covered by Cochiti Reservoir. Miocene–early Pliocene erosion (Manley, 1979) upstream in the Española Basin may record regional incision driven Incisional and Aggradational History of by climate change or local headward migration of a knick zone driven by fault-controlled subsidence of the La Bajada the Ancestral Rio Grande constriction area. The gravel-transporting ancestral Rio Grande extended at least as far south as the southern end of Exposures of coarse-grained alluvium in the La Bajada the Cochiti Pueblo map area (pl. 2) during late Miocene–early constriction area record deposition by the late Cenozoic Rio Pliocene time. Grande in the axial part of the basin and by tributary streams Geologic relations from adjacent basins help provide a on adjacent piedmonts (fig. B5). These fluvial deposits also context for interpreting upper Cenozoic deposits of the La provide evidence of depositional control by faulting, episodes Bajada constriction area. North of the area, previous workers of dramatically increased sediment load, and damming by (Spiegel and Baldwin, 1963; Griggs, 1964; Galusha and Blick, lava flows and sediment. Thickness of the composite basin-fill 1971; Manley, 1979) noted unconformities below and above stratigraphic section exceeds 300 m and locally may approach the upper Miocene Chamita Formation of the Española Basin 1,200 m (Smith, McIntosh, and Kuhle, 2001), particularly in and interpreted the upper unconformity in terms of tectonic the vicinity of Cochiti Dam. Distribution and continuity of deformation and establishment of an axial drainage. Deter- late Miocene through late Pliocene alluvial deposits and their mination of the timing of the following two important events inferred subsurface extent imply mainly basin aggradation dur- in the time-stratigraphic evolution of the Rio Grande basins ing this time in the La Bajada constriction area. To the south in depends on stratigraphic interpretations of upper Cenozoic the northern Albuquerque Basin, Lozinsky and others (1991) northern-provenance axial gravel deposits within the basin-fill and Connell and others (2001) interpreted nearly continuous sequence: cessation of “closed” basin deposition, and integra- basin filling in the eastern part of the basin from late Miocene tion of the Rio Grande as an axial system (Connell, 2001, through early Pleistocene time, whereas Williams and Cole 2004). It is difficult to determine, however, when either of (2005) hypothesize that the hydraulic regime changed and the these events occurred in the La Bajada constriction. Correla- western basin margin eroded during latest Miocene, probably tion of deposits within the upper part of the basin fill is ham- in response to climate change. pered by shallow level of exposure, multiple cycles of erosion Despite dominant aggradation in the area, axial and and deposition that recycled sedimentary rocks of similar type, tributary streams may have incised considerably in the area and difficulty in determining whether changes from aggrada- of the La Bajada constriction during intervals in late Miocene tion to erosion are local or regional in extent. Distinguishing and early Pliocene time. The substantial volume of coarse a change from deposition to long-term incision by the Rio alluvium near Cochiti Dam requires extensive erosion in the Grande would provide a convenient stratigraphic criterion for upstream drainage basin and bed-load transport by an ener- separating Santa Fe Group from post-Santa Fe Group deposits. getic axial river. Near White Rock Canyon, the ancestral Rio However, in the La Bajada constriction area, even where rift Grande cut down >250 m through slightly lithified sediments basin-fill sediments are deeply dissected and well exposed, it of the Santa Fe Group between about 8.4 and 2.9 Ma (Reneau is difficult to conclude that evidence of post-early Pleistocene, and Dethier, 1996). Downcutting by the Rio Grande and latest erosion represents the initial change to basin-scale incision. Upper Cenozoic Fluvial Deposits, La Bajada Constriction Area, New Mexico 37

Axial river gravel deposits yield geochronologic the ancestral Rio Grande flowed several kilometers east of evidence (see earlier discussion) of a through-flowing ances- Cochiti Reservoir, at an elevation lower than present river tral Rio Grande in the western part of the La Bajada constric- level, before migrating westward to the vicinity of Cochiti tion as early as about 7 Ma (fig. B5). Quartzite-rich sediment Reservoir and aggrading about 75 m before about 1.2 Ma of inferred late Miocene(?) age presumably derived from a (stratigraphic section DN–96–26, fig. B4C; fig. B5). distant source to the north was described from subsurface Erosion by the Rio Grande after 1.22 Ma rapidly drill cuttings along the western margin of the Española Basin removed deposits of the upper (Tshirege) Member of the (Purtymun, 1995), although this finding has not been cor- Bandelier Tuff (unit Qbt, pl. 2) that filled portions of the roborated by more recent work. Dacitic tephra interlayered lower part of White Rock Canyon and its tributary valleys. with northern-source axial river gravel deposits that has been Resulting tuffaceous debris accumulated in early(?) Pleis- 40 39 dated by Ar/ Ar techniques at about 5 Ma (WoldeGabriel tocene alluvial terrace-fill deposits (unit Qta1) that subse- and others, 2001) is exposed near the base of the Puye For- quently were incised by the Rio Grande. The next youngest mation in the western part of the Española Basin. Older(?) alluvial fill (unit Qta2) records a separate episode of aggrada- coarse gravels also are present in the subsurface beneath tion and incision during middle Pleistocene time (550–250 the Pajarito plateau (Reneau and Dethier, 1996; Rogers and ka), and some of the younger basin fill sediments record Smartt, 1996), but it is not known if they were deposited by similar aggradational episodes (Dethier, 1999). Periodic the ancestral Rio Grande (WoldeGabriel and others, 2001). cessation of middle and late Pleistocene incision followed by Reneau and Dethier (1996) interpreted subsurface data and aggradation and terrace development in the La Bajada con- exposures near White Rock Canyon as evidence that before striction area suggests that discharge and sediment load were the middle Pliocene the Rio Grande flowed west of its pres- linked to climatic cycles or to other events that intensified ent course and that downcutting began during latest Miocene after early Pleistocene time. Alluvial terrace fills younger or early Pliocene time. than late middle Pleistocene (unit Qta3) also record lateral Demonstrably uppermost Miocene basin-fill deposits asymmetry of sediment preservation south of Cochiti Dam are exposed only in the southwest part of the La Bajada con- (pl. 2), likely reflecting basinal tilt due to fault movements striction area, and no exposures of early Pliocene deposits (Smith, McIntosh, and Kuhle, 2001) or westward growth of have been identified in the area. Erosional unconformities the eastern piedmont deposits (or both). resulting from base-level changes are preserved within mid- dle Pliocene alluvial deposits at localities near Cochiti Res- ervoir. Basalt dated at about 2.9 Ma (unit Tb, pl. 2) discon- formably overlies Rio Grande axial gravel alluvium at river Hydrogeologic Implications level northeast of the reservoir. A separate, undated basalt flow (also unit Tb) also disconformably overlies Rio Grande Pliocene and younger ancestral Rio Grande axial gravel gravel some 60 m higher and 1,500 m west of the river-level and sand are the most important aquifer material in the sub- basalt. Also, 2.5-Ma flows are present about 30 m lower than surface of the Rio Grande valley (Smith and Kuhle, 1998b) the undated basalt flow around Cochiti Reservoir where they owing to their high porosity and permeability and great disconformably overlie axial gravel and lacustrine deposits. subsurface volume. Surface gravels may serve as a recharge During the interval 2.5–1.6 Ma, the Rio Grande cut a canyon zone where they are hydraulically connected with underly- 170 m deep in basalt at Water Canyon about 15 km north of ing aquifers. Near-surface Holocene coarse alluvium in the Cochiti Reservoir (pl. 1) (Reneau and Dethier, 1996). Such La Bajada constriction area is only tens of meters thick and episodes of Pliocene and Pleistocene downcutting in White lies above the water table, but Pleistocene and older gravel Rock Canyon north of the La Bajada constriction reflect in the subsurface is locally more than a hundred meters a knickpoint that migrated upstream, driven by faulting or thick and may have considerable lateral extent. Coarse regional downcutting (or both). alluvial deposits of similar age form critical portions of the In addition to changes in base level, Rio Grande allu- aquifer in the Albuquerque Basin to the south (for example, vial deposits record considerable lateral migration of the Kernodle, McAda, and Thorn, 1995). It seems likely that axial drainage from one side of the basin to the other (fig. pre-middle Pliocene gravel submerged beneath Cochiti B5). Smith, McIntosh, and Kuhle (2001) interpreted a late Reservoir for more than two decades promotes recharge of Pliocene eastward shift in river position and alluvial deposi- the Rio Grande valley aquifer southward from the Cochiti tion as a fluvial response to eastward tilting of the Santo Pueblo area. Lateral seepage of water into agricultural lands Domingo Basin. Alternatively, this eastward shift of the river of Cochiti Pueblo and Peña Blanca from Cochiti Dam pro- may record one or more of three other events: (1) block- vides evidence of this process, which has required substan- age by lava flows and maar deposits (Self and others, 1996) tial engineering mitigation (Blanchard and others, 1993). near the north end of Cochiti Reservoir; (2) upland erosion Geophysical data and interpretation (chapters D, F, and G, and expansion of the western piedmont fans; and (3) drain- this volume) provide additional constraints regarding the age changes due to headward cutting by the Rio Grande subsurface extent and configuration of coarse alluvial depos- in response to climate change or faulting. At about 1.8 Ma its in the La Bajada constriction area. 38 Hydrogeologic Framework, La Bajada Constriction Area, Rio Grande Rift, New Mexico

References Cited Connell, S.D., Cather, S.M., Dunbar, N.W., McIntosh, W.C., and Peters, L., 2002, Stratigraphy of the Tanos and Black- Aby, S.B., 1997, The terraces of Cochiti Canyon—Soil devel- share Formations (lower Santa Fe Group), Hagan embay- opment and relation to activity in the Pajarito fault zone: ment, Rio Grande rift, New Mexico: New Mexico Geology, Albuquerque, University of New Mexico, M.S. thesis, 159 p. v. 24, p. 107–117. Bartolino, J.R., and Cole, J.C., 2002, Ground-water resources Connell, S.D., Koning, D.J., and Cather, S.M., 1999, Revisions of the middle Rio Grande Basin, New Mexico: U.S. Geo- to the stratigraphic nomenclature of the Santa Fe Group, logical Survey Circular 1222, 132 p. northwestern Albuquerque Basin, New Mexico: New Mex- ico Geological Society 50th field conference, Albuquerque Blanchard, P.J., 1993, Ground-water-level fluctuations in the Geology, Guidebook, p. 337–353. Cochiti Dam–Peña Blanca area, Sandoval County, 1976–89: U.S. Geological Survey Water-Resources Investigations Connell, S.D., Love, D.W., and Lucas, S.G., eds., 2001, Report 92–4193, 72 p. Stratigraphy and tectonic development of the Albuquerque Basin, central Rio Grande rift—Mini-papers in field trip Bryan, Kirk, 1938, Geology and ground-water conditions of guidebook for the Geological Society of America Rocky the Rio Grande depression in Colorado and New Mexico, Mountain–South Central section meeting, Albuquerque, in U.S. National Resources Planning Board, The Rio New Mexico: New Mexico Bureau of Mines and Mineral Grande joint investigation in the upper Rio Grande Basin: Resources Open-file Report 454B, 93 p. Washington, D.C., U.S. Government Printing Office, v. 1, pt. 2, p. 197–225. Dethier, D.P., 1999, Quaternary evolution of the Rio Grande near Cochiti Lake, northeast Santo Domingo Basin, New Bryan, Kirk, and McCann, F.T., 1937, The Ceja del Rio Mexico: New Mexico Geological Society 50th field confer- Puerco—A border feature of the Basin and Range province ence, Albuquerque Geology, Guidebook, p. 371–378. in New Mexico, pt. 1, Stratigraphy and structure: Journal of Geology, v. 45, p. 801–828. Dethier, D.P., and McCoy, W.D., 1993, Aminostratigraphic relations and age of Quaternary deposits, northern Espa- Bryan, Kirk, and McCann, F.T., 1938, The Ceja del Rio ñola Basin, New Mexico: Quaternary Research, v. 39, Puerco—A border feature of the Basin and Range province p. 222–230. in New Mexico, pt. 2, Geomorphology: Journal of Geology, v. 46, p. 1–16. Galusha, Ted, and Blick, J.C., 1971, Stratigraphy of the Santa Fe Group, New Mexico: Bulletin of the American Museum Chamberlin, R.M., Jackson, P.B., and Connell, S.D., 1999, of Natural History, v. 144, 127 p. Field logs of borehole drilled for monitoring well at Dome Road site, Cañada de Cochiti Grant, Sandoval County, Griggs, R.L., 1964, Geology and groundwater resources of New Mexico: New Mexico Bureau of Mines and Mineral the Los Alamos area, New Mexico: U.S. Geological Survey Resources Open–file Report 444c, 30 p. Water-Supply Paper 1735, 107 p.

Chapin, C.E., and Cather, S.M., 1994, Tectonic setting of the Izett, G.A., and Obradovich, J.D., 1994, 40Ar/39Ar age axial basins of the northern and central Rio Grande rift, constraints for the Jaramillo normal subchron and the in Keller, G.R., and Cather, S.M., eds., Basins of the Rio Matuyama-Brunhes geomagnetic boundary: Journal of Geo- Grande rift—Structure, stratigraphy, and tectonic setting: physical Research, v. 99 (B2), p. 2925–2934. Geological Society of America Special Paper 294, p. 5–25. Kernodle, J.M., McAda, D.P., and Thorn, C.R., 1995, Simula- Connell, S.D., 2001, Stratigraphy of the Albuquerque Basin, tion of ground-water flow in the Albuquerque Basin, central Rio Grande rift, New Mexico—A progress report, A–1, in New Mexico, 1901–1994, with projections to 2020: U.S Connell, S.D., Love, D.W., and Lucas, S.G., eds., Stratig- Geological Survey Water-Resources Investigations Report raphy and tectonic development of the Albuquerque Basin, 94–4251, 114 p. central Rio Grande rift—Mini-papers in field trip guidebook for the Geological Society of America Rocky Mountain– Koning, D.J., Connell, S.D., Pazzaglia, F.J., and McIntosh, South Central section meeting, Albuquerque, New Mexico: W.C., 2002, Redefinition of the Ancha Formation and New Mexico Bureau of Mines and Mineral Resources Plio-Pleistocene deposition in the Santa Fe embayment, Open-file Report 454B, p. A1–A27. north-central New Mexico: New Mexico Geology, v. 24, p. 75–87. Connell, S.D., 2004, Geology of the Albuquerque Basin and tectonic development of the Rio Grande rift in north-central Kuhle, A.J., and Smith, G.A., 2001, Alluvial-slope deposi- New Mexico, in Mack, G.H., and Giles, K.A., eds., The tion of the Skull Ridge Member of the Tesuque Formation, geology of New Mexico—A geologic history: New Mexico Española Basin, New Mexico: New Mexico Geology, v. 23, Geological Society Special Publication 11, p. 359–388. p. 30–37. Upper Cenozoic Fluvial Deposits, La Bajada Constriction Area, New Mexico 39

Lanphere, M.A., Champion, D.E., Christiansen, R.L., Izett, Mexico, in Mack, G. H., and Giles, K.A., eds., The geology G.A., and Obradovich, J.D., 2002, Revised ages for tuffs of of New Mexico—A geologic history: New Mexico Geologi- the Yellowstone Plateau volcanic field—Assignment of the cal Society Special Publication 11, p. 331–358. Huckleberry Ridge Tuff to a new geomagnetic polarity event: Geological Society of America Bulletin, v. 114, p. 559–568. Smith, G.A., and Kuhle, A.J., 1998a, Geologic map of the Santo Domingo Pueblo quadrangle, Sandoval County, Lozinsky, R.P., Hawley, J.W., and Love, D.W., 1991, Geologic New Mexico: New Mexico Bureau of Mines and Min- overview and Pliocene-Quaternary history of the Albuquer- eral Resources, Open-file Digital Map OF–DM 26, scale que Basin, central New Mexico: New Mexico Bureau of 1:24,000. Mines and Mineral Resources Bulletin 137, p. 157–162. Smith, G.A., and Kuhle, A.J., 1998b, Geologic map of the Machette, M.N., 1978, Geologic map of the San Acacia Santo Domingo Pueblo SW quadrangle, Sandoval County, quadrangle, Socorro County, New Mexico: U.S. Geological New Mexico: New Mexico Bureau of Mines and Mineral Survey Map GQ–1415, scale 1:24,000. Resources Open-file Digital Map OF–DM 26, 1 sheet and text, scale 1:24,000. Manley, K., 1979, Stratigraphy and structure of the Española Basin, Rio Grande rift, New Mexico, in Riecker, R.E., ed., Smith, G.A., and Kuhle, A.J., 1998c, Hydrostratigraphic Rio Grande rift—Tectonics and magmatism: Washington, implications of new geologic mapping in the Santo D.C., American Geophysical Union, p. 71–86. Domingo Basin, New Mexico: New Mexico Geology, v. 20, p. 21–27. Purtymun, W.D., 1995, Geologic and hydrologic records of observation wells, test holes, test wells, supply wells, Smith, G.A., and Lavine, Alex, 1996, What is the Cochiti For- springs and surface water in the Los Alamos area: Los Ala- mation?: New Mexico Geological Society 47th field confer- mos National Laboratory Report LA–12883–MS, 339 p. ence, Jemez Mountains Region, Guidebook, p. 219–224. Reneau, S.L., and Dethier, D.P., 1996, Pliocene and Quater- Smith, G.A., McIntosh, William, and Kuhle, A.J., 2001, Sedi- nary history of the Rio Grande, White Rock Canyon and mentologic and geomorphic evidence for seesaw subsidence vicinity, New Mexico: New Mexico Geological Society of the Santo Domingo accommodation-zone basin, Rio 47th field conference, Jemez Mountains Region, Guide- Grande rift, New Mexico: Geological Society of America book, p. 317–324. Bulletin, v.113, p. 561–574. Reneau, S.L., Gardner, J.N., and Forman, S.L., 1996, New Spiegel, Zane, and Baldwin, Brewster, 1963, Geology and evidence for the age of the youngest eruptions in the Valles water resources of the Santa Fe area, New Mexico: U.S. caldera, New Mexico: Geology, v. 24, p. 7–10. Geological Survey Water-Supply Paper 1525, 258 p. Rogers, J.B., and Smartt, R.A., 1996, Climatic influences on Stearns, C.E., 1953, Tertiary geology of the Galisteo-Tonque Quaternary alluvial stratigraphy and terrace formation in the area, New Mexico: Geological Society of America Bulletin, Jemez River valley, New Mexico: New Mexico Geological v. 64, p. 459–508. Society 47th field conference, Jemez Mountains Region, Guidebook, p. 347–356. Tedford, R.H., 1981, Mammalian biochronology of the late Cenozoic basins of New Mexico: Geological Society of Sarna-Wojcicki, A.M., Morrison, S.D., Meyer, C.E., and America Bulletin, pt. 1, v. 92, p. 1008–1022. Hillhouse, J.W., 1987, Correlation of upper Cenozoic tephra layers between sediments of the western United States and Tedford, R.H., and Barghoorn, E.S., 1997, Miocene mammals comparison with biostratigraphic age data: Geological Soci- of the Espanola and Albuquerque Basins, north-central ety of America Bulletin, v. 98, p. 207–223. New Mexico, in Lucas, S.G., Estep, J.W., Williamson, T.E., and Morgan, S., eds., New Mexico’s fossil record 1: New Sawyer, D.A., Shroba, R.R., Minor, S.A., and Thompson, Mexico Museum of Natural History and Science Bulletin R.A., 2002, Geologic map of the Tetilla Peak quadrangle, 11, p. 77–95. Santa Fe and Sandoval Counties, New Mexico: U.S. Geo- logical Survey Miscellaneous Field Studies Map MF–2352, Tedford, R.H., and Barghoorn, E.S., 1999, Santa Fe Group scale 1:24,000. (Neogene), Ceja del Rio Puerco, northwestern Albuquerque Basin, Sandoval County, New Mexico: New Mexico Geo- Self, Stephen, Heiken, Grant, Sykes, M.L., Wohletz, Kenneth, logical Society 50th field conference, Albuquerque Geol- Fisher, R.V., and Dethier, D.P., 1996, Field excursions to ogy, Guidebook, p. 327–335. the Jemez Mountains, New Mexico: New Mexico Bureau of Mines and Mineral Resources Bulletin 134, 72 p. Williams, P.L., and Cole, J.C., 2005, Preliminary geologic map of the Albuquerque 30′ × 60′ quadrangle, north-central Smith, G.A., 2004, Middle to Late Cenozoic development of New Mexico: U.S. Geological Survey Open-File Report the Rio Grande rift and adjacent regions in northern New 2005–1418, 64 p., scale 1:100,000. 40 Hydrogeologic Framework, La Bajada Constriction Area, Rio Grande Rift, New Mexico

WoldeGabriel, Giday, Warren, R.G., Broxton, D.E., Vaniman, D.T., Heizler, M.T., Kluk, E.C., and Peters, L., 2001, Epi- sodic volcanism, petrology, and lithostratigraphy of the Paja- rito Plateau and adjacent areas of the Española Basin and the Jemez Mountains, in Crumpler, L.S., and Lucas, S.G., eds., Volcanology in New Mexico: New Mexico Museum of Natural History and Science Bulletin 18, p. 97–129.

Published in the Central Region, Denver, Colo. Manuscript approved for publication February 10, 2006 Graphics by authors Photocomposition by Mari L. Kauffmann (Contractor, ATA Services) Edited by Mary-Margaret Coates (Contractor, ATA Services)